Lubricating oil composition

ABSTRACT

A lubricating oil composition for internal combustion engines which composition is phosphorus-free is provided. The lubricating oil composition contains at least 60% wt. of base oil selected from Group I, Group II, Group III and Group IV base oils or mixtures thereof, and at least 1.4% wt. of one or more antioxidants selected from aminic antioxidants and/or phenolic antioxidants, based on the total weight of the lubricating oil composition.

FIELD OF THE INVENTION

The present invention relates to a lubricating oil composition.

BACKGROUND OF THE INVENTION

One of the most critical areas for a lubricating oil composition in aninternal combustion engine is the sliding cam/follower contact in thevalve train. A variety of methods for eliminating the cam mechanism havebeen proposed, with electrical and hydraulic actuation being two of themost popular.

The elimination of the cam actuation for the valves is not for thebenefit of the lubricating oil composition, but in order to have muchbetter control over the valve operation. These systems will allowindependent control of opening, closing and maximum lift of the inletand exhaust valves, in each individual cylinder of a multi-cylinderengine.

This opens the possibility for a wide range of engine improvements;improved exhaust emissions, improved torque curve and lower fuelconsumption, for example. Unusual operating modes are possible, such ascylinder cut-out and eight- or twelve-stroke cycle, which can lead toimproved performance as well.

SUMMARY OF THE INVENTION

A lubricating oil composition useful for internal combustion engines isprovided which composition is phosphorus-free, said compositioncomprising at least 60% wt. of base oil selected from the groupconsisting of Group I, Group II, Group III and Group IV base oils ormixtures thereof, and at least 1.4% wt. of at least one antioxidantselected from the group consisting of aminic antioxidants, phenolicantioxidants and mixtures thereof, based on the total weight of thelubricating oil composition.

A method of lubricating an internal engine using such lubricating oilcomposition is also provided.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, a lubricating oil composition which is suitable forlubricating camless internal combustion engines is provided.

Examples of “camless” internal combustion engines are disclosed in U.S.Pat. Nos. 5,255,641; 5,311,711; 5,367,990; 5,373,817; 5,377,631;5,404,844; 5,419,301; 5,456,221; 5,456,222; 5,562,070; 5,572,961;5,615,646; 5,619,965; 5,694,893; 5,709,178; 5,758,625; 5,970,956; and6,024,060 which disclosures are herein incorporated by reference.

The benefit of these systems for the lubricating oil composition is thatno special anti-wear additive is required for valve train protection.Hence, organometallic phosphorus antiwear additives such as zincdithiophosphate (ZnDTP) are not required. Simply making this change inisolation not only leads to a reduction in treat cost but also givesbenefits in other areas.

For example, ZnDTP contributes to the sulphated ash content, sulphurcontent and phosphorus content in a lubricating oil composition. In viewof the adverse affects that the sulphated ash, sulphur and phosphorusconcentrations of lubricating oil compositions may have on vehicleexhaust after-treatment devices, it may be desirable to developlubricating oil compositions with reduced sulphated ash, sulphur and/orphosphorus concentrations therein. Thus, reducing or avoiding the use ofZnDTP in lubricating oil compositions is a method to reduce the sulphurand phosphorus content therein.

In addition, the anti-wear films developed by ZnDTPs may cause increasedfriction, leading to extra power loss.

Accordingly, WO-A-02/24843 describes a method of operating a camlessinternal combustion engine which comprises:

-   -   (A) operating said engine using a normally liquid or gaseous        fuel composition; and    -   (B) lubricating said engine using a low-phosphorus or        phosphorus-free lubricating oil composition, said low-phosphorus        or phosphorus-free lubricating oil composition optionally        containing an extreme-pressure additive comprised of metal and        phosphorus, provided the amount of phosphorus contributed to        said low-phosphorus or phosphorus-free lubricating oil        composition by said extreme-pressure additive does not exceed        about 0.08% by weight based on the weight of said low-phosphorus        or phosphorus-free lubricating oil composition.

However, the removal of ZnDTP from lubricating oil compositions can leadto increased cylinder bore wear.

EP-A-1338643 describes reduced phosphorus lubricating oil compositionsfor use as passenger car engine lubricants.

The compositions comprise a major amount of Group II-IV and ester baseoils; an overbased calcium or magnesium salicylate detergent; anoil-soluble organo-molybdenum compound; an ashless dispersant; and asupplemental antioxidant. The supplemental antioxidant is said to reducethe tendency of base oils to deteriorate in service which deteriorationcan be evidenced by the products of oxidation such as sludge andvarnish-like deposits on metal surfaces and by viscosity growth. Whilstthe supplemental antioxidant is to be present in an amount of from 0.1to 5.0 wt %, the supplemental antioxidant is preferably present in anamount of 0.25 to 1.0 wt %. In this regard, the Example in EP-A-1338643employs supplemental antioxidant in an amount of 0.50% wt.

U.S. Pat. No. 5,439,605 describes various low ash and light ashformulations for use as motor oils. The base oils present in theformulations may optionally contain antioxidant in an amount from about0.5% to about 1.0%. In some embodiments, the formulations are indicatedto not contain phosphorus.

However, neither EP-A-1338643 nor U.S. Pat. No. 5,439,605 are concernedwith the development of phosphorus-free lubricating oils compositionsfor camless engines which have advantageous anti-wear properties, inparticular giving reduced cylinder bore wear.

It has been found in the present invention, lubricating oil compositionssuitable for camless engines which exhibit advantageous anti-wearproperties, in particular giving reduced cylinder bore wear. It isdesirable to develop phosphorus-free lubricating oil compositions forcamless engines which have beneficial anti-wear properties.

In one embodiment, the present invention provides a lubricating oilcomposition for internal combustion engines which composition isphosphorus-free and which composition comprises at least 60% wt. of baseoil, wherein said base oil is selected from Group I, Group II, Group IIIand Group IV base oils or mixtures thereof, and at least 1.4% wt. of oneor more antioxidants selected from the group of aminic antioxidantsand/or phenolic antioxidants, based on the total weight of thelubricating oil composition.

By “phosphorus-free” in the present invention, is meant that thelubricating oil composition does not comprise any phosphorus-containingcompounds therein.

In a preferred embodiment of the present invention, said one or moreantioxidants are present in an amount of at least 1.6% wt., morepreferably in an amount of at least 1.7% wt., based on the total weightof the lubricating oil composition.

The lubricating oil composition of the present invention may compriseone or more aminic antioxidants.

Examples of aminic antioxidants which may be conveniently used includealkylated diphenylamines, phenyl-α-naphthylamines,phenyl-β-naphthylamines and alkylated α-naphthylamines.

Preferred aminic antioxidants include dialkyldiphenylamines such asp,p′-dioctyl-diphenylamine, p,p′-di-α-methylbenzyl-diphenylamine andN-p-butylphenyl-N-p′-octylphenylamine, monoalkyldiphenylamines such asmono-t-butyldiphenylamine and mono-octyldiphenylamine,bis(dialkylphenyl)amines such as di-(2,4-diethylphenyl)amine anddi(2-ethyl-4-nonylphenyl)amine, alkylphenyl-1-naphthylamines such asoctylphenyl-1-naphthylamine and n-t-dodecylphenyl-1-naphthylamine,1-naphthylamine, arylnaphthylamines such as phenyl-1-naphthylamine,phenyl-2-naphthylamine, N-hexylphenyl-2-naphthylamine andN-octylphenyl-2-naphthylamine, phenylenediamines such asN,N′-diisopropyl-p-phenylenediamine andN,N′-diphenyl-p-phenylenediamine, and phenothiazines such asphenothiazine and 3,7-dioctylphenothiazine.

Preferred aminic antioxidants include those available under thefollowing trade designations: “Sonoflex OD-3” (ex. Seiko Kagaku Co.),“Irganox L-57” (ex. Ciba Specialty Chemicals Co.) and phenothiazine (ex.Hodogaya Kagaku Co.).

The lubricating oil composition of the present invention may compriseone or more phenolic antioxidants.

Examples of phenolic antioxidants which may be conveniently used includeC7-C9 branched alkyl esters of3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy-benzenepropanoic acid,2-t-butylphenol, 2-t-butyl-4-methylphenol, 2-t-butyl-5-methylphenol,2,4-di-t-butylphenol, 2,4-dimethyl-6-t-butylphenol,2-t-butyl-4-methoxyphenol, 3-t-butyl-4-methoxyphenol,2,5-di-t-butylhydroquinone, 2,6-di-t-butyl-4-alkylphenols such as2,6-di-t-butylphenol, 2,6-di-t-butyl-4-methylphenol and2,6-di-t-butyl-4-ethylphenol, 2,6-di-t-butyl-4-alkoxyphenols such as2,6-di-t-butyl-4-methoxyphenol and 2,6-di-t-butyl-4-ethoxyphenol,3,5-di-t-butyl-4-hydroxybenzylmercaptooctylacetate,alkyl-3-(3,5-di-t-butyl-4hydroxyphenyl)propionates such asn-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,n-butyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and2′-ethylhexyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,2,6-d-t-butyl-α-dimethylamino-p-cresol,2,2′-methylenebis(4-alkyl-6-t-butylphenol) such as2,2′-methylenebis(4-methyl-6-t-butylphenol, and2,2-methylenebis(4-ethyl-6-t-butylphenol), bisphenols such as4,4′-butylidenebis(3-methyl-6-t-butylphenol,4,4′-methylenebis(2,6-di-t-butylphenol), 4,4′-bis(2,6-di-t-butylphenol),2,2-(di-p-hydroxyphenyl)propane,2,2-bis(3,5-di-t-butyl-4-hydroxyphenyl)propane,4,4′-cyclohexylidenebis(2,6-t-butylphenol),hexamethyleneglycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],triethyleneglycolbis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate],2,2′-thio-[diethyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],3,9-bis{1,1-dimethyl-2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)-propionyloxy]ethyl}2,4,8,10-tetraoxaspiro[5,5]undecane,4,4′-thiobis(3-methyl-6-t-butylphenol) and2,2′-thiobis(4,6-di-t-butylresorcinol), polyphenols such astetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,bis-[3,3′-bis(4′-hydroxy-3′-t-butylphenyl)butyric acid]glycol ester,2-(3′,5′-di-t-butyl-4-hydroxyphenyl)methyl-4-(2″,4″-di-t-butyl-3″-hydroxyphenyl)methyl-6-t-butylphenoland 2,6-bis(2′-hydroxy-3′-t-butyl-5′-methylbenzyl)-4-methylphenol, andp-t-butylphenol-formaldehyde condensates andp-t-butylphenol-acetaldehyde condensates.

Preferred phenolic antioxidants include those available under thefollowing trade designations: “Irganox L-135” (ex. Ciba SpecialtyChemicals Co.), “Anteeji DBH” (ex. Kawaguchi Kagaku Co.,), “Yoshinox SS”(ex. Yoshitomi Seiyaku Co.), “Antage W-400” (ex. Kawaguchi Kagaku Co.),“Antage W-500” (ex. Kawaguchi Kagaku Co.), “Antage W-300” (ex. KawaguchiKagaku Co.), “Ionox 220AH” (ex. Shell Japan Co.), bisphenol A, producedby the Shell Japan Co., “Irganox L109” (ex. Ciba Speciality ChemicalsCo.), “Tominox 917” (ex. Yoshitomi Seiyaku Co.), “Irganox L115” (ex.Ciba Speciality Chemicals Co.), “Sumilizer GA80” (ex. Sumitomo Kagaku),“Antage RC” (ex. Kawaguchi Kagaku Co.), “Irganox L101” (ex. CibaSpeciality Chemicals Co.), “Yoshinox 930” (ex. Yoshitomi Seiyaku Co.),“Ionox 330” (ex. Shell Japan Co.).

The amount of base oil incorporated in the lubricating oil compositionof the present invention is preferably present in an amount of at least60% wt., more preferably in an amount in the range of from 60 to 98%wt., most preferably in an amount in the range of from 75 to 90% wt.,with respect to the total weight of the lubricating oil composition.

By “Group I” base oil, “Group II” base oil, “Group III” base oil and“Group IV” base oil in the present invention are meant base oilsaccording to the definitions of American Petroleum Institute (API)categories I, II, III and IV. Such API categories are defined in APIPublication 1509, 15^(th) Edition, Appendix E, April 2002.

Group I base oils contain less than 90% saturates (according to ASTMD2007) and/or greater than 0.03% sulphur (according to ASTM D2622,D4294, D4927 or D3120) and have a viscosity index of greater than orequal to 80 and less than 120 (according to ASTM D2270).

Group II base oils contain greater than or equal to 90% saturates andless than or equal to 0.03% sulphur and have a viscosity index ofgreater than or equal to 80 and less than 120, according to theaforementioned ASTM methods.

Group III base oils contain greater than or equal to 90% saturates andless than or equal to 0.03% sulphur and have a viscosity index ofgreater than 120, according to the afore-mentioned ASTM methods.

Group IV base oils are polyalphaolefins (PAO).

There are no particular limitations regarding the Group I to IV baseoils used in the present invention, and various conventional known GroupI to IV base oils selected from mineral oils and synthetic lubricatingoils may be conveniently used.

Mineral oils include liquid petroleum oils and solvent-treated oracid-treated mineral lubricating oil of the paraffinic, naphthenic, ormixed paraffinic/naphthenic type which may be further refined byhydrofinishing processes and/or dewaxing.

Naphthenic base oils have low viscosity index (VI) (generally 40-80) anda low pour point. Such base oils are produced from feedstocks rich innaphthenes and low in wax content and are used mainly for lubricants inwhich colour and colour stability are important, and VI and oxidationstability are of secondary importance.

Paraffinic base oils have higher VI (generally >95) and a high pourpoint. Such base oils are produced from feedstocks rich in paraffins,and are used for lubricants in which VI and oxidation stability areimportant.

Fischer-Tropsch derived base oils may be used as the base oil in thelubricating oil composition of the present invention, for example, theFischer-Tropsch derived base oils disclosed in EP-A-776959, EP-A-668342,WO-A-97/21788, WO-00/15736, WO-00/14188, WO-00/14187, WO-00/14183,WO-00/14179, WO-00/08115, WO-99/41332, EP-1029029, WO-01/18156 andWO-01/57166.

Synthetic processes enable molecules to be built from simpler substancesor to have their structures modified to give the precise propertiesrequired.

Synthetic lubricating oils include hydrocarbon oils such as olefinoligomers (PAOs) (Group IV base oils) and dewaxed waxy raffinate.

Synthetic Group III hydrocarbon base oils sold by the Royal Dutch/ShellGroup of Companies under the designation “XHVI” (trade mark) may beused.

Preferably, the base oil used in the present invention is constitutedfrom mineral oils and/or synthetic base oils which contain more than 80%wt of saturates, preferably more than 90% wt., as measured according toASTM D2007.

It is further preferred that the base oil used in the present inventioncontains less than 1.0% wt., preferably less than 0.1% wt. of sulphur,calculated as elemental sulphur and measured according to ASTM D2622,ASTM D4294, ASTM D4927 or ASTM D3120.

Preferably, the viscosity index of base oil used in the presentinvention is more than 80, more preferably more than 120, as measuredaccording to ASTM D2270.

Preferably, the base oil used in the present invention has a kinematicviscosity in the range of from 2 to 80 mm²/s at 100° C., more preferablyof from 3 to 70 mm²/s, most preferably of from 4 to 50 mm²/s.

The lubricating oil composition of the present invention preferably hasa sulphated ash content of not greater than 1.3 wt. %, more preferablynot greater than 1.1 wt. % and most preferably not greater than 1.0 wt.%, based on the total weight of the lubricating oil composition.

The lubricating oil composition of the present invention preferably hasa sulphur content of not greater than 1.2 wt. %, more preferably notgreater than 0.8 wt. % and most preferably not greater than 0.05 wt. %,based on the total weight of the lubricating oil composition.

The lubricating oil composition of the present invention may furthercomprise additional additives such as anti-wear additives, detergents,dispersants, friction modifiers, viscosity index improvers, pour pointdepressants, corrosion inhibitors, defoaming agents and seal fix or sealcompatibility agents, provided that said additive components do notcontain any phosphorus therein.

Suitable phosphorus-free anti-wear additives include boron-containingcompounds such as borate esters, borated fatty amines, borated epoxides,alkali metal (or mixed alkali or alkaline earth metal) borates andborated overbased metal salts.

The boron-containing anti-wear additives may be added to the lubricatingoil composition of the present invention in an amount in the range offrom 0.1 to 3.0 wt. %, based on the total weight of lubricating oilcomposition.

Typical detergents that may be used in the lubricating oil of thepresent invention include one or more salicylate and/or phenate and/orsulphonate detergents.

However, as metal organic and inorganic base salts which are used asdetergents can contribute to the sulphated ash content of a lubricatingcomposition, in a preferred embodiment of the present invention, theamounts of such additives are minimised.

The salicylate and/or phenate and/or sulphonate detergents may be addedin an amount in the range of from 0.01 to 20.0 wt. %, more preferablyfrom 0.10 to 10.0 wt. %, based on the total weight of lubricating oilcomposition.

In order to maintain a low sulphur level, salicylate detergents arepreferred.

Thus, in a preferred embodiment, the lubricating oil composition of thepresent invention may contain one or more salicylate detergents.

It is preferred that the salicylate and/or phenate and/or sulphonatedetergents, independently, have a TBN (total base number) in the rangeof from 10 to 500 mg.KOH/g, more preferably in the range of from 30 to350 mg.KOH/g and most preferably in the range of from 50 to 300mg.KOH/g, as measured by ASTM D2894.

The lubricating oil compositions of the present invention mayadditionally contain an ash-free dispersant which is preferably admixedin an amount in the range of from 5 to 15% wt., based on the totalweight of the lubricating oil composition.

Typical dispersants that may be conveniently employed in the lubricatingoil composition of the present invention, include ash-free alkenyl- oralkyl-succinimides and polyalkenyl succininic acid esters or derivativesthereof. Such ash-free dispersants may be borated. The dispersants mayhave a high molecular weight (for example, of greater than 2000) or alow molecular weight (for example, of less than 2000, preferably lessthan 1200).

Dispersants that may be employed in the lubricating oil composition ofthe present invention include those described in EP-A-1167497 andJapanese Patent Nos. 1367796, 1667140, 1302811 and 1743435.

Preferred friction modifiers that may be used include fatty acid amides,more preferably unsaturated fatty acid amides. The total amount ofunsaturated fatty acid amide compound added is preferably from 0.05 to0.35% wt., based on the total weight of the lubricating oil composition.

Examples of viscosity index improvers which may be used in thelubricating oil composition of the present invention include thestyrene-butadiene copolymers, styrene-isoprene stellate copolymers andthe polymethacrylate-based and ethylene-propylene copolymers and thelike disclosed in Japanese Patent Nos. 954077, 1031507, 1468752, 1764494and 1751082. Such viscosity index improvers may be conveniently employedin an amount in the range of from 1 to 20% wt., based on the totalweight of the lubricating oil composition. Similarly, dispersing-typeviscosity index improvers comprising copolymerized polar monomercontaining nitrogen atoms and oxygen atoms in the molecule may also beused therein.

Polymethacrylates such as, for example, those as disclosed in JapanesePatent Nos. 1195542 and 1264056 may be employed in the lubricating oilcompositions of the present invention as effective pour pointdepressants.

Furthermore, compounds such as alkenyl succinic acid or ester moietiesthereof, benzotriazole-based compounds and thiodiazole-based compoundsmay be used in the lubricating oil composition of the present inventionas corrosion inhibitors.

Compounds such as, for example, dimethyl polycyclohexane, polyacrylatemay be used in the lubricating oil composition of the present inventionas defoaming agents.

Compounds which may be used in the lubricating oil composition of thepresent invention as seal fix or seal compatibility agents include, forexample, commercially available aromatic esters.

The lubricating oil compositions of the present invention may beprepared by admixing the antioxidants selected from the group of aminicantioxidants and/or phenolic antioxidants, and, optionally, one or morefurther additives that are usually present in lubricating oils, forexample as herein before described, with a mineral and/or synthetic baseoil.

Lubricating oil compositions of the present invention display reducedwear, in particular reduced cylinder bore wear. Accordingly, in afurther embodiment of the present invention, there is provided the useof a lubricating oil composition as hereinbefore described to reducewear, preferably cylinder bore wear, in an internal combustion engine,in particular in a camless internal combustion engine.

In another embodiment of the present invention, there is provided amethod of lubricating an internal combustion engine, in particular acamless internal combustion engine comprising applying a lubricating oilcomposition as hereinbefore described thereto.

The present invention is described below with reference to the followingExamples, which are not intended to limit the scope of the invention inany way.

EXAMPLES

Formulations

Tables 1 and 2 indicate the formulations that were tested.

Standard conventional detergents, dispersants, pour point depressantsand viscosity modifiers were used therein.

The aminic antioxidant used was that available under the tradedesignation “Irganox L-57” ex. Ciba Specialty Chemicals Co.(p,p′-dioctyl-diphenylamine).

The phenolic antioxidant used was that available under the tradedesignation “Irganox L-135” ex. Ciba Speciality Chemicals Co. (octyl3-(3,5-di-t-butyl-4-hydroxylphenyl)propionate).

Apart from the formulations of Examples 2 and 3, all of the formulationsdescribed in Tables 1 and 2 were nominally SAE 10W40 viscosity gradeoils, as they were minor formulations variants of the baseline oil.

The formulations tested in Examples 2 and 3 were SAE 5W30 viscositygrade oils.

Table 3 indicates the physical characteristics of the formulations thatwere tested. TABLE 1 Comparative Comparative Additive Example 1 Example2 Example 1 Example 2 Antifoam 30 ppm 30 ppm 30 ppm 30 ppmMagnesium/Calcium 3.85 3.85 3.85  3.85 Detergents (% wt.) Succinimide7.5 7.5 7.5 7.5 Ashless Dispersant (% wt.) Aminic Antioxidant 1.7 1.70.3 0.3 (% wt.) Zinc — — 1.0 — dithiophosphate (% wt.) Pour point 0.20.2 0.2 0.2 depressant (% wt.) Viscosity index 18.5 11 18.5 18.5modifier concentrate (A) ⁽¹⁾ (% wt.) Base Oil HVI-105 ⁽²⁾ 5 — 5 5 (%wt.) HVI-65 ⁽³⁾ 53.25 19.75 53.65 54.65 (% wt.) XHVI-5.2 ⁽⁴⁾ 10 56 10 10(% wt.) TOTAL 100 100 100 100⁽¹⁾ Hydrogenated styrene-isoprene viscosity index modifier (6%) and pourpoint depressant (1.9%) in diluent oil.⁽²⁾ Group I base oil.⁽³⁾ Group II base oil.⁽⁴⁾ Group III base oil

TABLE 2 Additive Example 3 Example 4 Antifoam 793 ppm 793 ppmMagnesium/Calcium Detergents (% wt.) 2.20 2.20 Ashless Dispersant ⁽¹⁾ (%wt.) 8.00 8.00 Phenolic Antioxidant (% wt.) 4.00 4.0 Viscosity IndexModifier 2.90 — concentrate (B) ⁽²⁾ (% wt.) Viscosity Index Modifier2.90 — concentrate (C) ⁽³⁾ (% wt.) Dispersant-Viscosity Index Modifier2.00 2.00 (% wt.) Supplementary Additive Package ⁽⁴⁾ 1.00 1.00 (% wt.)Base Oil PAO ⁽⁵⁾ 5.00 5.00 XHVI - 8.0 ⁽⁶⁾ (% wt.) — 50.00 XHVI - 5.2 ⁽⁷⁾(% wt.) 67.00 27.80 XHVI - 4.0 ⁽⁸⁾ (% wt.) 5.00 — TOTAL 100⁽¹⁾ PIB-MALA polyamine dispersant.⁽²⁾ Hydrogenated styrene-isoprene viscosity index modifier (6%) indiluent oil.⁽³⁾ Hydrogenated polyisoprene viscosity index modifier (15%) in diluentoil.⁽⁴⁾ Supplementary additive package containing a mixture of conventionaladditives:- friction modifier, corrosion inhibitor and seal fix agent.⁽⁵⁾ Group IV base oil.^((6), (7), (8)) Group III base oils.

TABLE 3 Comp. Comp. Analytical Results Example 1 Example 2 Example 3Example 4 Example 1 Example 2 Vk*, 100° C. (cSt) 14.74 10.24 11.90 11.6714.44 14.46 Vk*, 40° C. (cSt) 98.07 58.01 66.93 70.87 94.99 94.37 CCS**,−25° C. 7730 6269 @ −30° C. 6237 @ −30° C. 5959 7304 6995 (mPa · s)(ASTM D5293) Mg (% wt. ) 0.104 0.101 0.074 0.077 0.104 0.104 (ICP-OESmethod) Ca (% wt.) 0.129 0.12 0.029 0.03 0.127 0.13 (ICP-OES Method) Zn(% wt.) — — — — 0.106 — (ICP-OES method) P (% wt.) — — — — 0.101 —(ICP-OES method)*Kinematic Viscosity**Low temperature Cranking ViscosityScreening Rig Test

In order to test the lubricating oil compositions described in Tables 1to 3, screening rig tests were used to simulate a camless engineenvironment.

In said screening rigs, the oil system for the valve train was separatedform the rest of the internal combustion engine, thereby allowing thelower part of the internal combustion engine to be lubricated with a“camless” type lubricant as it did not come into contact with thecam-follower area.

A screening rig test was developed by Shell Global Solutions (UK) usinga Renault Megane 1.6 litre gasoline engine. The test cycle mimickedshort distance stop-start driving, in a similar manner to the ASTMSequence V test.

The Renault Megane 1.6 litre gasoline engine was modified to separatethe oil supply and return to the overhead camshaft valve train in thecylinder head from their normal routes in and out of the cylinder block.The cylinder head was lubricated by a separate external electric pump ina circuit with reservoir and temperature control. A conventionalfully-formulated lubricant was used in this circuit.

The test oil was used in the remainder of the original engine circuitbelow the head. The modifications were confined to the cylinder head, sothat it could be re-used. It was possible to use the head for severaltest runs before it required overhauling. A new short motor was used foreach test, which was stripped and measured beforehand, and the pistonring gaps were increased to give higher blow-by flow rate for increasedseverity.

The standard test duration was 288 hours (12 days), running on a fourhour cycle. The engine was stopped once a day for oil sampling andlevelling/top-up.

At the end of test, the engine was dismantled and components measuredfor wear calculations.

The engine used for the second series of tests was a Ford Zetec SE 1.7litre DOHC, as used in the Puma Coupe. It was modified in a similarmanner to the Megane engine in order to simulate camless operation. Therest of the test installation and method was the same as for the Meganetest, except that all the tests were run to a duration of 576 hours (24days).

The screening rig testing is a publicly available commercial testingservice that is available ex. Shell Global Solutions (UK), CheshireInnovation Park, P.O. Box 1, Chester CH1 3SH, UK.

Results and Discussion

Some of the formulations described in Tables 1 and 2 were tested usingthe afore-mentioned screening rig tests and the results obtained thereonare included in the following tables.

(i) Screening rig using Renault Megane 1.6 Litre Gasoline Engine

A standard test duration of 288 hours was used (i.e. the same durationas the ASTM Sequence VE test upon which the screening rig test wasbased) for the results in Table 4. TABLE 4 Average Thrust Side Bore WearStep Depth at Top Ring Reversal (μm) Example 1 2.0 Example 2 7.6Comparative Example 1* 2.6 Comparative Example 2 9.7*Average of two test runs

It is evident from the above results for Comparative Example 2 that whenZnDTP is removed from the baseline lubricating oil composition ofComparative Example 1, there is an increase in the depth of the wearstep in the cylinder bore at top ring reversal (TRR).

However, it is apparent from Example 1 that the addition of a greatlyincreased amount of antioxidant not only reduces the bore wear stepdepth to levels below those observed in Comparative Example 2, but saidincreased amount of antioxidant also surprisingly reduces the thrustside bore wear step depth to levels below that observed in theZnDTP-containing formulation of Comparative Example 1.

In addition, the formulations of Examples 1 and 2 were also tested underan extended test duration of 576 hours. The results for the extendedtesting are given in Table 5 below. TABLE 5 Average Thrust Side BoreWear Step Depth at top ring reversal (μm) Example 1* 5.85 Example 2*20.85 Comparative Example 1** 2.6 Comparative Example 2** 9.7*576 hours test duration**288 hours test duration

It is further evident from the above results that even when theformulation of Example 1 was tested for an extended period, the borewear depth was still much lower than for the formulation of ComparativeExample 2 which has much lower amounts of antioxidant therein.

(ii) Screening Rig Using Ford Zetec SE 1.7 Litre DOHC Engine

As stated above, the test operational details for this rig were the sameas those used with the Megane engine, except that all tests were run tothe double duration (576 hours).

The wear step measurement technique was slightly different, in that thetotal cross-sectional area of the material removed in the wear step wasmeasured, rather than the maximum depth of the step.

Also the information is for the anti-thrust side, rather than thethrust. TABLE 6 Average Anti-thrust Side Bore Wear Area (top 20 mm ofcylinder liner) (μm²) Example 2 357.5 × 10³ Example 3 328.2 × 10³Example 4 243.3 × 10³

As can be seen above in (i), the formulation of Example 2 exhibitsadvantageous anti-wear properties. Table 6 indicates the totalcross-sectional area of material removed in the wear step of a screeningrig using Ford Zetec SE 1.7 litre DOHC engine using the formulation ofExample 2.

It is apparent from Table 6 that the formulations of Example 3 and 4(which comprised phenolic antioxidant rather than aminic antioxidant)had even lower bore wear than the formulation of Example 2.

In addition, by comparing the results in Table 6 for Examples 3 and 4,it is apparent that the higher viscosity grade formulation (i.e. theformulation of Example 4) gives lower wear.

1. A lubricating oil composition useful for internal combustion engineswhich composition is phosphorus-free, said composition comprising atleast 60% wt. of base oil selected from the group consisting of Group I,Group II, Group III and Group IV base oils or mixtures thereof, and atleast 1.4% wt. of at least one antioxidant selected from the groupconsisting of aminic antioxidants, phenolic antioxidants and mixturesthereof, based on the total weight of the lubricating oil composition.2. The lubricating oil composition of claim 1 wherein said antioxidantis present in an amount of at least 1.6% wt., based on the total weightof the lubricating oil composition.
 3. The lubricating oil compositionof claim 1 wherein said antioxidant is an aminic antioxidant.
 4. Thelubricating oil composition of claim 3 wherein said aminic antioxidantis selected from the group consisting of alkylated diphenylamines,phenyl-α-naphthylamines, phenyl-β-naphthylamines and alkylatedα-naphthylamines.
 5. The lubricating oil of claim 2 wherein saidantioxidant is an aminic antioxidant.
 6. The lubricating oil compositionof claim 1 wherein said antioxidant is a phenolic antioxidant.
 7. Thelubricating oil composition of claim 5 wherein said phenolicantioxidants are selected from 2,6-di-t-butylphenol, C7-C9 branchedalkyl esters of 3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy-benzenepropanoicacid, and 4,4′-methylenebis(2,6-di-t-butylphenol).
 8. The lubricatingoil composition of claim 2 wherein said antioxidant is a phenolicantioxidant.
 9. A method of lubricating an internal combustion enginecomprising applying a lubricating oil composition of claim 1 to aportion of the internal combustion engine.
 10. The method of claim 9wherein the internal combustion engine is a camless internal combustionengine.